This invention relates to transient current sensing of keyboards and more particularly to keyboards of the capacitive touch-plate type.
Capacitive touch-plate keyboards and attendant sensing circuits have heretofore been known, illustrative of which those disclosed in U.S. Pat. Nos. 4,293,987 and 4,304,976 in the names of Tom L. Gottbreht and Glen C. Shepherd.
While capacitive touch switch panels have constituted marked improvements over the prior designs, and while the interrogating and sensing circuits have operated satisfactorily, the necessity of paralleling certain interrogating and sensing circuits to achieve reasonable economies has resulted in the requirement for very tight manufacturing and adjustment tolerances as will hereinafter be described in greater detail, thus adding to cost of manufacture and subsequent operational vulnerability.
Accordingly, it is one general object of this invention to improve capacitive touch switch panel sensing circuits.
It is another object of this invention to reduce the manufacturing costs of such circuits.
It is still another object of this invention to provide circuits having greater latitudes of operability, thereby permitting the relaxation of certain manufacturing tolerances and achieving improvement in cost.
It is still another object of this invention through the provision of greater tolerances, to render the circuits less vulnerable subsequently to malfunction.
Accordingly, and in accordance with one feature of the invention, a plurality of capacitive touch switches are connected to a common comparator circuit, and a source of constant voltage is connected to the sensing circuits, thereby permitting the utilization of transient current as the key activation representing condition.
In accordance with another feature of the invention, the transient current is sensed by a corresponding change in voltage at a voltage comparator.
In accordance with still another feature of the invention, a connection is advantageously made from the output of the voltage comparator to a selected one of the comparator inputs, thereby, in cooperative association with other circuit elements, providing a type of comparator hysteresis.
In accordance with still another feature of the invention, through the use of current sensing, a plurality of keys can be connected to a single sensor without substantially degrading circuit characteristics, and a relatively low voltage source can be used.
These and other objects and features of the invention will be apparent from the following description, by way of example, with reference to the drawing in which: